Impairment values are more often combined than added, and the Combined Values Chart in the AMA Guides to the Evaluation of Permanent Impairment can be used to calculate the combined value of two numbers. The values are derived from the formula A + B(1 – A) = the combined value of A and B, where A and B are the decimal equivalents of the impairment ratings. This mathematically prevents an estimate of impairment greater than 100%. With smaller numbers, the combined value may equal the arithmetic sum of the 2 numbers. Impairments of different organ systems are converted to whole person impairment ratings before combining. In most lower extremity impairment cases only one evaluation method is used, but certain circumstances justify combining impairments. Examples in the lower extremity include diagnosis-based estimates with short leg, degenerative joint disease (in the case of fractures in and about joints), and neurologic loss. Combining vs adding can be confusing, but one can nearly always combine, with the following exceptions for the musculoskeletal system: range of motion (ROM) deficits within an upper extremity; ROM deficits of the thumb; total hand impairment; rating hip or knee replacement results; and ROM deficits of the spine at a specific level.